Evaporators are used in a wide range of industries, such as Chemical, Mining, Food, Pharmaceuticals, etc. and are among the most expensive process units used in these industries. Evaporation of slurry requires massive amounts of energy in comparison to simple heating of the same slurry. Accordingly, the majority of evaporators are energy intensive. By some estimates, 25% of the total energy consumption of a plant takes place inside evaporators. While evaporators operating with electricity or hot water do exist in very limited numbers in some industries, by far the majority of evaporators operate with steam that has been generated by a steam generator.
In particular, most manufacturing plants utilize steam in a number of processes, including evaporation in particular. Therefore, most manufacturing plants utilize, typically, a number of steam generators, for example, housed in one area of the plant, which operate as a sort of “Utility Process Unit” to supply steam to the different manufacturing processes throughout the plant. Often, the steam can be used relatively efficiently in different process or stages depending on various temperature requirements of different processes. Additionally, it is typical for other processes of a plant to produce steam as waste heat, which can be combined and used with the steam generated by the steam generators. Moreover, because most manufacturing plants utilize water for a number of processes, water is typically available for the steam generators.
Cheremisinoff, “Handbook of Chemical Processing Equipment,” describes the prevalent use of steam and demonstrates that the assumption that steam should be used for evaporation systems (see, e.g., p. 95 in which evaporators are first classified into two main categories, “Steam Inside Tube” and “Steam Outside Tube”).
Minton, Paul E., “Handbook of Evaporation Technology,” Noyes Publications, Westwood, N.J., pp. 1-402 (1986) describes types of evaporators and applications. A number of challenges are discussed, including the problems of scaling (or crystallization on the metal surfaces of the heat-transfer tube of the evaporator), salting and fouling, which can especially occur in multi-effect evaporations systems and require shut-down of the evaporation process unit for cleaning or to replace the heat exchange tubes thereof. One reason that evaporators used in multiple-effect evaporation systems are more prone to scaling is because such evaporation systems typically operate with liquids higher in scale forming content. The use of energy-intensive forced circulation and agitation in short tube-type evaporators are some of the measures used to reduce the problem of scaling. The agitator in such an evaporator acts as a pump to circulate the flow inside the heat exchange tubes through which the slurry is provided. However, the major of circulation in such evaporators has roots in the change of feed density. Due to the size and a high number of tubes used to create a greater heat-exchange surface and limits on the impeller size and speed, agitation is not very effective. With the exception of tanks with steam coils agitation in other types of evaporators cannot be applied since the heat exchangers are of a shell and tube design allowing for no agitator installation. As a result, preventing evaporator scaling has been an ongoing challenge across many industries.